1. Field of the Invention
This invention relates to a method and apparatus for protecting an electrical switching element, illustratively, a thyristor, used in a pulse generator which supplies a capacitive load, against damage from spark-overs that may occur in that load.
2. Description of the Prior Art
An inherent problem in using pulsed energy to drive a large capacitive load, e.g. that present in large electrostatic precipitators, is that a considerable amount of energy is required to repetitively charge the load capacitance to a high voltage level. Since the capacitive load dissipates relatively little energy as the result of each applied pulse, recovery of the remaining energy stored in the load, and not consumed by, for example, corona discharge or arcing, is of prime economic importance.
This problem has been solved by using so-called energy conserving pulse generators. Arrangements which perform this function are well-known in the art and are typified by that described in British Pat. No. 1,544,105. There, energy conservation is obtained by connecting a diode in reverse across the thyristor, i.e. the diode's anode is connected to the thyristor's cathode, and vice versa. Specifically in this arrangement, the current flowing through the capacitive load as the result of any drive pulse takes on a sinusoidal waveshape. During one half of the sinusoid, the thyristor conducts; however, it fails to conduct during the second half. To conserve energy, the diode conducts the load current which flows during the second half and while, more particularly, the thyristor assumes a substantially non-conductive state.
The design criteria for the thyristors and diodes in the switch element are:
hold-off voltage
pulse width
peak on-state current
maximum rate of rise of current
thyristor turn-off time.
Spark-overs occur from time to time in certain capacitive loads and in fact often occur in precipitators. Specifically, spark-overs can occur both during the rise of an applied voltage pulse appearing across the precipitator and during the decay of that pulse. If a spark-over occurs during the decay, then the thyristor may be disadvantageously destroyed. Specifically, in the energy conserving pulse system described in British Pat. No. 1,544,105, the load current, as discussed above, flows through the diode while the thyristor is in a substantially non-conductive state. During a part of the time while this current is flowing, the thyristor is recovering its hold-off strength. The hold-off strength is defined as the thyristor's ability to block current flow while withstanding a forward biased voltage applied across its anode and cathode. While the thyristor is recovering, the magnitude of the current flowing through the thyristor continually decreases. Unfortunately, during this time, the thyristor has an unevenly distributed conductivity. Should spark-over occur any time during recovery, then a substantial pulse of current will flow through the thyristor and will, more particularly, be concentrated in those spots having high conductivity. This, in turn, may cause excessive heating at those spots which may disadvantageously destroy the thyristor.